In order to study topological magnon excitations, we consider a linear spin-wave theory approach in the low temperature regime where magnon excitations are well-defined. Higher order interaction effects will not change the main conclusions of our article. The energy bands were obtained by diagonalizing the Fourier transformed Hamiltonian as outlined in the Methods and Supplementary Information. While the magnon picture is effective at low temperatures and offers explanation of experimental data [1]}, it fails at higher temperatures where the ground state is not purely ferromagnetic. The approximation \(\sqrt{2 S-b^{\dag }b}\rightarrow \sqrt{2 S}\)  becomes invalid. Consequently, we utilize the Schwinger boson mean-field theory (SBMFT) [2]} formalism for the thermally disordered quantum paramagnetic regime [3]}. The Schwinger boson approach is capable of describing experimental results [4]} and can additionally give rise to the spin Nernst effect [3]}. Calculation details on the SBMFT application are provided in Supplementary Information.
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